Exhaust systems for high-performance internal combustion engines of the type used on racing vehicles have been the subject of empirical design work and theoretical studies. One area of focus is the reflection of pressure waves at a change in the cross-sectional area of the exhaust system piping, such as found at the merge location where two or more primary header pipes are combined into a secondary header pipe. It has been discovered that pressure wave reflections can be applied in a manner that enhances engine performance. For instance, exhaust pulses from the engine can be reflected cyclically back to the engine exhaust port of the same or of an adjacent power cylinder as an expansion or rarefaction wave, so that for a portion of each cycle the exhaust pulse can assist with scavenging of the exhaust of the engine cylinder to increase the horsepower output of the engine.
Timing or “tuning” the reflected pressure waves to reach the exhaust port at the right moment, however, can be particularly difficult. This is because the propagation speed of the pressure pulse varies significantly with the temperature and composition of the media or gas through which it travels, which is difficult to determine in an exhaust system, and because the exhaust headers are not filled with a homogeneous density or pressure of gas. Thus, calculations usually are based upon a plurality of assumptions or approximations which seldom correlate with the reality of conditions inside an exhaust header pipe. As a practical matter, therefore, it is extremely difficult to obtain significant horsepower improvement using this technique because the horsepower increases occur only at very precise, and often unpredictable, engine speeds.
Another common misconception with high-performance exhaust systems is that engine performance may also be increased by reducing the cross-sectional area of the exhaust system piping at a location slightly downstream of the merge location of two or more primary headers, to create a converging-diverging choke point that accelerates the exhaust gases. It is thought that this velocity increase can fluidly couple the header pipes and create a Venturi effect which scavenges the exhaust gases from the inactive header pipe(s). However, studies have shown that this configuration will only be effective within a certain engine rpm band when the multiple exhaust gas pulses being sequentially discharged from each primary header are adequately spaced so as to not interfere with the others, and will progressively become ineffective at off-timing speeds as the multiple gas pulses begin to obstruct one another and create a constriction at the choke point. Additionally, expansion in the primary header pipes and subsequent recompression at the venturi choke point is detrimental to exhaust scavenging.